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       access - check user’s permissions for a file


       #include <unistd.h>

       int access(const char *pathname, int mode);


       access()  checks whether the process would be allowed to read, write or
       test for existence of the file (or other file system object) whose name
       is  pathname.   If  pathname is a symbolic link permissions of the file
       referred to by this symbolic link are tested.

       mode is a mask consisting of one or more of R_OK, W_OK, X_OK and  F_OK.

       R_OK,  W_OK  and  X_OK request checking whether the file exists and has
       read, write and execute permissions, respectively.  F_OK just  requests
       checking for the existence of the file.

       The tests depend on the permissions of the directories occurring in the
       path to the file, as given in  pathname,  and  on  the  permissions  of
       directories  and files referred to by symbolic links encountered on the

       The check is done with the process’s real UID and GID, rather than with
       the  effective  IDs  as  is done when actually attempting an operation.
       This is to allow set-user-ID programs to easily determine the  invoking
       user’s authority.

       Only   access  bits  are  checked,  not  the  file  type  or  contents.
       Therefore, if a directory is found to be "writable," it probably  means
       that  files can be created in the directory, and not that the directory
       can be written as a file.  Similarly, a DOS file may  be  found  to  be
       "executable," but the execve(2) call will still fail.

       If  the  process  has  appropriate  privileges,  an  implementation may
       indicate success for X_OK even if none of the execute  file  permission
       bits are set.


       On  success  (all requested permissions granted), zero is returned.  On
       error (at least one bit in mode asked for a permission that is  denied,
       or  some  other  error  occurred),  -1  is  returned,  and errno is set


       access() shall fail if:

       EACCES The requested access would be  denied  to  the  file  or  search
              permission  is  denied  for  one  of the directories in the path
              prefix of pathname.  (See also path_resolution(2).)

       ELOOP  Too many symbolic links were encountered in resolving  pathname.

              pathname is too long.

       ENOENT A directory component in pathname would have been accessible but
              does not exist or was a dangling symbolic link.

              A component used as a directory in pathname is not, in  fact,  a

       EROFS  Write  permission  was  requested  for  a  file  on  a read-only

       access() may fail if:

       EFAULT pathname points outside your accessible address space.

       EINVAL mode was incorrectly specified.

       EIO    An I/O error occurred.

       ENOMEM Insufficient kernel memory was available.

              Write access was requested  to  an  executable  which  is  being


       access()  returns  an error if any of the access types in the requested
       call fails, even if other types might be successful.

       access() may not work correctly on NFS file systems  with  UID  mapping
       enabled,  because UID mapping is done on the server and hidden from the
       client, which checks permissions.

       Using access() to check if a user is authorized to  e.g.  open  a  file
       before actually doing so using open(2) creates a security hole, because
       the user might exploit the short time  interval  between  checking  and
       opening the file to manipulate it.


       SVID, AT&T, POSIX, X/OPEN, 4.3BSD


       chmod(2),  chown(2), open(2), path_resolution(2), setgid(2), setuid(2),